1. Field of the Invention
The present invention relates to a toner adhesion amount measuring apparatus, and a toner adhesion amount measuring method, for measuring a toner adhesion amount in a toner image formed on an image bearing member of an image forming apparatus.
2. Description of the Related Art
A color of an image formed by an image forming apparatus using an electrophotographic process may be varied due to change of various physical parameters, even if a setting of the apparatus during an image formation is unchanged. In particular, developing/transfer processes have much influence on variation of color. Because, latent image potential, amount of toner applied, transfer efficiency, and the like are varied according to environmental variations of temperature/humidity and the like, and thus an amount of toner adhered onto a photosensitive drum and a transfer belt is not stable.
Thus, an amount of toner adhered onto the photosensitive drum or onto the transfer belt is to be measured to perform feedback controls over exposure amount, developing voltage, transfer current and the like, based on the measurement results, to stabilize the developing/transfer processes.
Generally, these controls are performed at the time points when variation of printer environment occurs. The variation of the printer environment occurs at the time points after toner cartridge replacement, after printing of predetermined number of sheets, and after power of printer body is turned on. When a toner adhesion amount is measured, a plurality of toner patches of various densities ranging from a low density to a high density are formed on a drum or a belt. Then, the toner adhesion amounts of these patches are measured by the toner adhesion amount measuring apparatus, and then various controls are performed under appropriate image forming conditions based on the measurement results.
Japanese Patent Application Laid-Open No. 62-280869 discusses a method for detecting reflected light amount when light is irradiated onto an image bearing member, and reflected light amount when light is irradiated onto a toner patch, measuring a toner adhesion amount using a difference of these reflected light amounts, thereby controlling image density parameters based on the measured values.
In a case where the toner adhesion amount is detected by the reflected light amounts, an average toner adhesion amount of the entire toner patch is measured by irradiating a light with a spot diameter larger than a screen period of the toner patch, and by totally detecting the reflected light from the entire toner patch.
Japanese Patent Application Laid-Open No. 8-327331 and Japanese Patent Application Laid-Open No. 9-68830 discuss a method for detecting a toner adhesion amount by measuring thickness of a toner patch (layer thickness) by a laser displacement gauge. The method includes irradiating a spot light onto an image bearing member and a toner image, causing the reflected light to form an image at a position depending on the layer thickness of the toner patch adhered onto the image bearing member, measuring the toner adhesion amount based on change of image-formed position, and performing feedback control of image density parameters of an imaging system based on result of layer thickness measurement.
For a toner patch having a screen structure, the toner adhesion amount is determined by measuring sectional profile (line height and line width) of screen lines, by scanning irradiated light with a spot diameter smaller than a screen period, onto the toner patch.
To scan the irradiated light onto the toner patch, there is a method to rotate a roller by a motor or the like, to move the toner patch in a horizontal direction together with the image bearing member, while the toner adhesion amount measuring apparatus is fixed within the image forming apparatus.
However, there is a backlash in a driving system for rotating the motor and the roller. As a result, even if driving of the image bearing member and measurement are started at the same timing at each time, the ranges to be actually measured may not always coincide with each other.
Further, since installation position of the toner adhesion amount measuring apparatus within the image forming apparatus also has a difference for each individual element, the measurement range varies also from one image forming apparatus to another.
There may be an issue that deviation of the measurement range causes errors in measured values of the toner patch having a periodicity in a screen.
As illustrated in FIG. 20, a reflection position signal and a reflected light amount signal from the toner patch included within the same measurement range vary between a case where a measurement starting position and an edge of the screen coincide with each other (phase 0), and a case where a measurement starting position and an edge of the screen deviate from each other (phase 1/4, −1/4). Therefore, the toner adhesion amounts calculated from these varying reflected signals also vary, and the variation of the adhesion amounts causes errors of measured values. As a result, the measurement accuracy of the toner adhesion amounts decreases.